Multifunctional nanocrystals

ABSTRACT

Multifunctional nanocomposites are provided including a core of either a magnetic material or an inorganic semiconductor, and, a shell of either a magnetic material or an inorganic semiconductor, wherein the core and the shell are of differing materials, such multifunctional nanocomposites having multifunctional properties including magnetic properties from the magnetic material and optical properties from the inorganic semiconductor material. Various applications of such multifunctional nanocomposites are also provided.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to all-inorganic multifunctionalnanocrystals, e.g., bifunctional nanocrystals, especially all-inorganicbifunctional nanocrystals.

BACKGROUND OF THE INVENTION

Nanocomposite materials provide the possibility for enhancedfunctionality and multi-functional properties in contrast withmore-limited single-component counterparts. One example of ananocomposite material is the inorganic core-shell structure. In thecase where semiconductors comprise the core and shell, the core-shellmotif has permitted enhanced photoluminescence, improved stabilityagainst photochemical oxidation, enhanced processibility, and engineeredband structures. Where metals have been combined in core-shellstructures, noble metals have been grown on magnetic metal cores and thereverse, for example, causing changes in magnetic, optical and chemicalproperties compared to the properties of the individual components.While examples of enhancement or modification of properties resultingfrom the core-shell structures are becoming more common, instances oftruly multifunctional behavior remain rare. For example, iron oxidenanoparticles overcoated with a dye-impregnated silica shell were shownto retain the magnetic properties of the core, while exhibiting theluminescent optical properties of the organic dye.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention provides a compositenanoparticle having multifunctional properties comprising a core of amaterial such as a magnetic material or an inorganic semiconductor; and,a shell of a material such as a magnetic material or an inorganicsemiconductor, the core and shell of differing materials and suchmultifunctional properties including magnetic properties from themagnetic material and optical properties from the inorganicsemiconductor material.

The present invention further provides a process of forming suchcomposite nanoparticles.

The present invention still further addresses uses or applications ofsuch composite nanoparticles including applications such as: an improveddetection/characterization of biomolecules by taking advantage ofmultifunctional properties of the composite nanocrystal, e.g., anoptical reporter function for detection coupled with a magnetic labelfor collection, where the ability to tune the blocking temperature ofthe magnetic component by altering nanocrystal surface propertiesimparts additional flexibility in applications by allowing finetemperature control over ferromagnetic-superparamagnetic phasetransition that can be used to control dispersibility of compositenanocrystal-labeled biomolecules; an improved asset label/tag; animproved source of spin-polarized electrons and holes for spininjectors; and, a component for magnetic field-modulated emitters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) shows a first embodiment of a core/shell nanocomposite inaccordance with the present invention; FIG. 1( b) shows a secondembodiment of a core/shell nanocomposite in accordance with the presentinvention; FIG. 1( c) shows an embodiment of a core/shell nanocompositewith a spacer layer in accordance with the present invention; and, FIG.1( d) shows another embodiment of a core/shell nanocomposite with aspacer layer in accordance with the present invention.

FIG. 2( a) shows a digital representation of a transmission electronmicrograph (TEM) image of Co/CdSe core/shell nanocomposites and FIG. 2(b) shows a digital representation of a high resolution TEM image of acomposite nanocrystal revealing the polycrystalline nature of thisshell.

FIG. 3 shows XRD patterns for Co nanocrystals and Co/CdSe core/shellnanocrystals compared to calculated patterns for ε-Co and wurtzite CdSe.

FIG. 4( a) shows temperature dependence of the magnetization for fieldcooled (open circles) and zero field cooled (filled circles) Conanocrystals and Co/CdSe core/shell nanocrystals (traces intersect atthe blocking temperature, the transition from superparamagnetic toferromagnetic behavior) and FIG. 4( b) shows the field dependence of themagnetization for the same samples.

FIG. 5( a) shows UV absorption and photoluminescence (PL) spectra ofCo/CdSe core/shell nanocomposites and FIG. 5( b) shows normalized PLdynamics taken at 20 K of CdSe nanocrystals (grey line) and Co/CdSecore/shell nanocrystals (dotted line), after subtracting thecontribution of the slow PL dynamics of the CdSe nanocrystals that aresynthesized simultaneously at a small fraction (observed as a minorsample component in TEM studies) with the Co/CdSe core/shellnanocrystals. The straight line is an exponential fit with a timeconstant of 0.7 nanoseconds (ns).

DETAILED DESCRIPTION

The present invention is concerned with multifunctional nanocompositesor multifunctional nanocrystals, e.g., bifunctional nanocrystals,especially all-inorganic bifunctional nanocrystals. In particular, thepresent invention is concerned with magnetic and luminescentnanocrystals. The multifunctional nanocomposites or multifunctionalnanocrystals of the present invention can be especially useful as labelsor tags in biological applications and the like due to the combinationof, e.g., magnetic properties and optical properties.

The present invention provides an all-inorganic multifunctionalnanocomposite. In one embodiment as shown in FIG. 1( a), such amultifunctional nanocomposite 10 includes a magnetic material core 12such as Co, and an inorganic semiconductor shell 14 such as CdSe. Thistype of structure, i.e., a Co core and a CdSe shell, is an example of anall-inorganic bi-functional nanoparticle, such a core/shell combinationpossessing both magnetic and luminescent properties within a singleall-inorganic quantum dot. In another embodiment as shown in FIG. 1( b),such a multifunctional nanocomposite 20 includes an inorganicsemiconductor core 22 such as CdSe, and a magnetic material shell 24such as Co. This type of structure, i.e., a CdSe core and a Co shell, isanother example of an all-inorganic bi-functional nanoparticlepossessing both magnetic and luminescent properties within a singleall-inorganic quantum dot. The present invention further provides a new,efficient source of spin-polarized electrons and holes for spintronicsapplications. The present invention still further providesfield-modulated nanocrystal emitters.

The nanocomposites are generally members of a crystalline populationhaving a narrow size distribution (standard deviation ≦20%), althoughthe size distribution can be broadened if desired. The shape of thenanocomposites can be a sphere, a rod, a wire, a disk, a branchedstructure and the like.

In one embodiment of the present invention, the nanocomposite includes amagnetic material core and an inorganic semiconductor shell, while inanother embodiment, the nanocomposite includes an inorganicsemiconductor core and a magnetic material shell. The magnetic materialcores or magnetic material shells can generally be of metals such ascobalt, nickel, iron, iron-platinum (FePt), an iron oxide such as, e.g.,Fe₂O₃ or Fe₃O₄, and a magnesium iron oxide spinel such as MgFe₂O₄ andthe like.

The shells in the present invention are generally uniform about theentirety of the core, i.e., a wholly complete shell is desired about thecore. Typically, cores will have at least one dimension between about1.5 nm and 30 nm. This complete shell is desired whether the core is ametallic material core or an inorganic semiconductor core. The shellscan be formed upon the core as either single crystalline materials or aspolycrystalline materials depending upon deposition conditions andlattice matching between the underlying material and the shell material.Generally, the shells of an active material will be from about 0.3 nm toabout 3 nm in thickness.

The optically active semiconductor cores or shells can be of aninorganic material selected from among Group II-VI compounds, Group II-Vcompounds, Group III-VI compounds, Group III-V compounds, Group IV-VIcompounds, Group I-III-VI compounds, Group II-IV-V compounds, and GroupII-IV-VI compounds. By “optically active” is meant that these materialscan absorb or emit light such optical properties dependent uponcomposition, location or size (dimension). Examples include cadmiumsulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), zincsulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), mercurysulfide (HgS), mercury selenide (HgSe), mercury telluride (HgTe),aluminum nitride (AIN), aluminum phosphide (AlP), aluminum arsenide(AlAs), aluminum antimonide (AlSb), gallium arsenide (GaAs), galliumnitride (GaN), gallium phosphide (GaP), gallium antimonide (GaSb),indium arsenide (InAs), indium nitride (InN), indium phosphide (InP),indium antimonide (InSb), thallium arsenide (TlAs), thallium nitride(TlN), thallium phosphide (TlP), thallium antimonide (TlSb), leadsulfide (PbS), lead selenide (PbSe), lead telluride (PbTe), zinc cadmiumselenide (ZnCdSe), indium gallium nitride (InGaN), indium galliumarsenide (InGaAs), indium gallium phosphide (InGaP), aluminum indiumnitride (AlInN), indium aluminum phosphide (InAlP), indium aluminumarsenide (InAlAs), aluminum gallium arsenide (AlGaAs), aluminum galliumphosphide (AlGaP), aluminum indium gallium arsenide (AlInGaAs), aluminumindium gallium nitride (AlInGaN) and the like, mixtures of suchmaterials, or any other semiconductor or similar materials. Theinorganic semiconductor shell upon the magnetic material core caninclude a single optically active inorganic semiconductor shell or caninclude multiple optically active inorganic semiconductor shells forselective tuning of the properties. Such multiple inorganicsemiconductor shells can be of differing inorganic semiconductormaterials. For example, a composite nanocrystal could include a cobaltcore, an active shell layer of CdSe and a second active shell layer ofCdTe.

Additionally, it can be important to surface passivate themultifunctional nanocomposites by overcoating the nanocomposites with ashell of a wide-gap semiconductor, e.g., zinc sulfide. Thus, where thereis a shell of an inorganic semiconductor material upon a magneticmaterial core, the inorganic semiconductor shell can have an overcoatingon the outer surface of the shell. Such an overcoating can also be asemiconductor material, such an overcoating having a compositiondifferent than the composition of the core, but generally having a bandgap that is larger than the band gap of the underlying inorganicsemiconductor shell material. The overcoat on the surface of themultifunctional nanocomposites can include materials selected from amongGroup II-VI compounds, Group II-V compounds, Group III-VI compounds,Group III-V compounds, Group IV-VI compounds, Group I-III-VI compounds,Group II-VI-VI compounds, and Group II-IV-V compounds. Examples includecadmium sulfide (CdS), cadmium telluride (CdTe), zinc sulfide (ZnS),zinc selenide (ZnSe), zinc telluride (ZnTe), mercury sulfide (HgS),mercury selenide (HgSe), mercury telluride (HgTe), aluminum nitride(AlN), aluminum phosphide (AlP), aluminum arsenide (AlAs), aluminumantimonide (AlSb), gallium arsenide (GaAs), gallium nitride (GaN),gallium phosphide (GaP), gallium antimonide (GaSb), indium arsenide(InAs), indium nitride (InN), indium phosphide (InP), indium antimonide(InSb), thalium arsenide (TlAs), thalium nitride (TlN), thaliumphosphide (TlP), thalium antimonide (TlSb), lead sulfide (PbS), leadselenide (PbSe), lead telluride (PbTe), mixtures of all such materials,or any other semiconductor or similar materials.

Suitable overcoatings can generally be applied as described in U.S. Pat.No. 6,322,901 by Bawendi et al. wherein overcoatings were applied tonanocrystalline quantum dots, such description incorporated herein byreference.

In one embodiment of the present invention as shown in FIG. 1( c), aspacer layer 36 is employed between a magnetic material core 32 and aninorganic semiconductor shell 34 of a multifunctional nanocomposite 30in order to minimize potential quenching of the shell properties by themagnetic material core. In another embodiment as shown in FIG. 1( d), aspacer layer 46 is employed between an inorganic semiconductor core 42and magnetic material shell 44 of multifunctional nanocomposite 40 inorder to minimize potential quenching of the core semiconductorproperties by the magnetic material shell. Such spacers can typically beof (a) inorganic semiconductors that are essentially functioning solelyas a spacer material without any contribution of electronic propertiesfrom that material to the overall structure or (b) dielectric materialssuch as silica and the like. The “non-active” spacer layer may alsofunction to minimize lattice strain between “active” layers.

The multifunctional nanocomposites of the present invention offer uniquepossibilities for bioassay labeling or tagging applications whereoptical properties (e.g., emission) of the shell facilitate opticaldetection of tagged biomolecules or physical assets, respectively, andthe magnetic properties of the core facilitate magnetic collection oftagged biomolecules or adherence of tags to certain physical assets,respectively. Further, the ability to tune certain properties such asblocking temperature (the transition from superparamagnetic toferromagnetic behavior), would allow, e.g., reversible sampleaggregation. That is, for sample temperatures below the blockingtemperature, the tag would be ferromagnetic, facilitating agglomerationunder an applied magnet. Raising the sample temperature above theblocking temperature would cause a transition to the superparamagneticstate and facilitate re-dissolution/separation of the taggedbiomolecules, facilitating further characterization of the sample.

The multifunctional nanocomposites of the present invention may alsooffer a unique efficient source of spin-polarized electrons and holesfor “semiconductor spintronics” applications, making them trulymultifunctional. Present day spin injectors, based on passing currentfrom a ferromagnetic contact into a semiconductor, are limited to lessthan about 10 percent efficiency due largely to conductivity mismatch atthe interface. Due to the uniquely intimate contact between a magneticand semiconducting material provided by the nano core/shell or nanosegmented structures, such hybrid nanocomposites may circumvent theproblems of current spin injectors.

The multifunctional nanocomposites of the present invention may alsoprovide magnetic-field-modulated emitters, where the photoluminescencefrom the semiconductor component is field tunable due to the influenceof the magnetic component on the semiconductor spin structure.

The present invention also provides a process of forming compositenanoparticles, each composite nanoparticle including a core of either amagnetic material or an inorganic semiconductor material and a shell ofeither a magnetic material or an inorganic semiconductor material, thecore and shell being of differing materials. The process involvessuspending or solvating nanoparticles of either a magnetic material oran inorganic semiconductor material within a liquid medium (suchnanoparticles being the core), introducing precursors for a shell ofeither a magnetic material or an inorganic semiconductor material intothe liquid medium; and, reacting the precursors under conditions capableof obtaining deposition wherein the shell is formed on the corenanoparticles. In the instance of Co nanoparticles as the core andcadmium selenide as the shell material, the conditions capable ofachieving deposition wherein the CdSe shell is formed on the Co corenanoparticles can be at reaction temperatures of from about 70° C. toabout 200° C., more preferably from about 120° C. to about 200° C. Forother systems, such optimal temperature ranges will likely vary betweenabout 70° C. to about 300° C. as can be readily determined by oneskilled in the art. Other conditions than temperature such as precursorconcentration, core material concentration, concentration of otherligands and addition methods of the materials may be determinative ofthe desired deposition as well.

A synthetic method has been developed for the preparation of trulybi-functional, all inorganic NCs that combine the properties of magneticnanoparticles and semiconductor quantum dots for the first time in acore/shell arrangement. While the nanocomposites retain the optical andmagnetic properties of the component parts, permitting potentialapplications that would make use of this novel bifunctionality, e.g.,optical “reporters” coupled with magnetic “handles” for use inbioassays, the respective properties are altered due to the uniquecore/shell structure.

The present invention is more particularly described in the followingexamples that are intended as illustrative only, since numerousmodifications and variations will be apparent to those skilled in theart.

EXAMPLE 1

Co/CdSe core/shell nanocomposites were prepared by controlled CdSedeposition onto preformed Co nanocrystals (NCs). The Co NCs weresynthesized by high temperature decomposition of organometallicprecursors, Co₂(CO)₂, in the presence of organic surfactant molecules.After the reaction, the Co NCs were precipitated by the addition of anon-solvent, anhydrous methanol, and re-dissolved in a nonpolar solventsuch as toluene or hexane. By repeating this process, the Co NCs wereeffectively “washed” and excess surfactant was removed. For thecore/shell preparation, washed Co NCs (2.7 mmole) were dispersed inn-Hexane (about 2 mL). Trioctylphosphine oxide (TOPO, 99%; 10 g) andhexadecylamine (HDA, 99%; 5 g) were then heated to 120° C. under vacuumin a reaction flask. After two hours, the TOPO and HDA were placed undernitrogen and heated to 140° C. A small portion of this mixture (about 1mL) was added to the Co NCs, and additional hexane was added if theresulting solution was very thick. This solution was then transferredback into the reaction flask. CdSe precursors [dimethylcadmium, 1.35mmole, and Se, 1.5 mmole dissolved in 1.5 mL trioctylphosphine (TOP), in5 mL additional TOP] were added dropwise into the vigorously stirredmixture. The reaction was held at temperature overnight. The lowreaction temperature (in comparison with a conventional CdSe synthesis)required a long incubation time. Further, higher temperatures (>200° C.)resulted in exclusively homogeneous nucleation and growth of CdSe NCs,unassociated with the Co NCs. While the lower-temperature preparationdid generate some fraction of both uncoated Co cores and unassociatedCdSe NCs, the various fractions were isolable using a combination ofstandard size-selective precipitation/washing steps followed by magneticseparations. In general, methanol was used to destabilize the solutions,resulting first in precipitation of Co cores (brown solid) that could beredissolved in dichlorobenzene. CdSe NCs and Co/CdSe core/shell NCs wereboth soluble in hexane but could be separated by size. Further, byplacing a magnet near a methanol-destabilized suspension, the core/shellNC component attracted to the magnet. The emission from the compositeparticles was easily seen when excited by a hand-held fluorescent lamp.

The core Co NCs are reasonably monodisperse (±15-20%) with a diameter ofabout 11 nm. The Co/CdSe core/shell NCs retain the spherical shape ofthe seed core and exhibit a uniform shell that is 2 to 3 nm thick (FIG.2( a) and FIG. 2( b)). The contrast between the Co-core and CdSe-shellis easily distinguishable by conventional TEM microscopy (FIG. 2( a)),with the precise nanostructure of the shell visible in high resolution(HR) imaging. As a possible mechanism for shell growth, we suggest arandom, highly non-epitaxial, nucleation of CdSe on the Co surfacefollowed by CdSe particle growth and nanocrystallite merging. The lowgrowth temperature used for the CdSe deposition likely supportsprimarily heterogeneous rather than homogeneous nucleation, and theuniformity of the shell suggests a sufficient annealing process to builda complete coating.

As determined by powder x-ray diffraction (XRD) (FIG. 3), the Co NCsgrow as the ε-Co phase, which is typical of the preparative methodemployed here. The Co/CdSe core/shell NCs yield an XRD pattern thatcontains additional diffraction peaks which can be indexed towurtzite-CdSe (confirmed in HR-TEM: about 2.2 Å and about 2.6 Å latticespacings match the (11) and (102) crystal planes of wurtzite CdSe). Theadded broadness of the XRD reflections in the composite-structurepattern results from the very small domain size characteristic of thepolycrystalline CdSe shell.

DC magnetization as a function of temperature in an applied magneticfield of 100 Oe was recorded for the Co and the Co/CdSe NCs (FIG. 4(a)). For 11 nm ε-Co NCs, the blocking temperature, T_(B), is above 350K, but the transition from super-paramagnetic to ferromagnetic behaviorafter CDSE-shell coating occurs at approximately 240 K (FIG. 4( a)).Since no significant change in Co-core size and shape was observed inTEM, the decrease in blocking temperature was observed whenmagnetic-optical nanocrystals were prepared as dimmers. The coercivity,the strength of a demagnetizing field required to coerce a magneticparticle to change magnetization direction, H_(c), was also determinedand found to be nearly the same for both samples, 0.11 Tesla (FIG. 4(b)), although there is a large drop in saturation magnetization per gramin the core/shell structures due to the presence of the nonmagnetic CdSephase. The coercivity of single-domain NCs depends mainly on themagnet-crystalline anisotropy and the domain size of the particles. Theconsistency in coercivity between the two samples correlates well withTEM observations that magnetic-core particle size did not changeappreciably. Further, it indicates that the coercivity is determinedmainly by magneto-crystalline anisotropy, rather than surface anisotropywhich would be sensitive to surface modification.

Absorption and emission spectra of the core/shell nanocomposites arepresented in FIG. 5( a). The observations of a relatively large Stokesshift further distinguishes the core-shell NCs from pure CdSe quantumdots. Monodisperse CdSe nanoparticle solutions of similarly sized NQDsexhibit a Stokes shift of about 20 nm, compared to the nanocomposites'40-50 nm shift. While the large Stokes shift may be related to theeffect of the presence of a close-proximity nanomagnet on thesemiconductor optical properties, it can also be attributed to CdSeshape anisotropy. Specifically, the crystallite domains visible inhigh-resolution TEM (FIG. 2( b)) are approximately 2×3 nm in size. Theabsorption edge roughly correlates with a CdSe NC having thesedimensions, but the photoluminescence (PS) maximum is shifted. Likely,pairs of neighboring domains are sufficiently well associated such thatthey behave as single “nanorods” causing the observed Stokesshift—similar to CdSe nanorod samples, where the Stokes shift is largecompared to approximately spherical particles. For the Co/CdSe NCs weobtain a quantum yield (QY) in emission of about 2 to 3%. While notoptimized, this is comparable to QYs (5 to 6%) obtained for CdSeprepared by similar preparative routes without, for example, ZnSovercoating to enhance emission efficiency. In addition, it was foundthat the PL dynamics of the Co/CdSe NCs are distinctly different fromthat for CdSe NCs (FIG. 5( b)). At low temperatures (20K), wheretrapping of excited carriers is strongly reduced, and therefore PLdynamics of NCs is normally dominated by relatively slow radiative decay(time constant >50 ns), we observe that the PL of the core/shell NCsdecays very rapidly, within less than 1 nanosecond. These preliminaryresults suggest that the accelerated PL decay is the result of CdSeshell emission quenching the presence of the metallic Co core, althoughit may also result from a modified exciton spin structure induced bymagnetic interactions.

Although the present invention has been described with reference tospecific details, it is not intended that such details should beregarded as limitations upon the scope of the invention, except as andto the extent that they are included in the accompanying claims.

1. A composite nanoparticle comprising: a core of a material selectedfrom the group consisting of a magnetic material and an inorganicsemiconductor; and, a shell of a material selected from the groupconsisting of an inorganic semiconductor and a magnetic material,wherein said core and said shell are of differing materials and saidcomposite nanoparticle is characterized as having multifunctionalproperties including magnetic properties from said magnetic material andoptical properties from said inorganic semiconductor material.
 2. Thecomposite nanoparticle of claim 1 including a multiplicity ofnanoparticles wherein said cores have a monodisperse size distribution.3. The composite nanoparticle of claim 1 wherein said core includesmultiple layers of at least two active inorganic semiconductors.
 4. Thecomposite nanoparticle of claim 1 wherein said shell includes multiplelayers of at least two active inorganic semiconductors.
 5. The compositenanoparticle of claim 1 wherein said core has at least one dimension ofbetween about 1.5 nm and about 30 nm.
 6. The composite nanoparticle ofclaim 1 wherein said shell is from about 0.3 nm to about 3 nm inthickness.
 7. The composite nanoparticle of claim 1 wherein said shellis single crystalline or polycrystalline.
 8. The composite nanoparticleof claim 1 wherein said core has a structure selected from the groupconsisting of spheres, rods, wires, and branched structures.
 9. Thecomposite nanoparticle of claim 1 wherein said core is of a magneticmaterial selected from the group consisting of cobalt, nickel, iron,iron-platinum, iron oxide, and a magnesium iron oxide spinel.
 10. Thecomposite nanoparticle of claim 1 wherein said core is of an inorganicsemiconductor material selected from the group consisting of Group II-VIcompounds, Group II-V compounds, Group III-VI compounds, Group III-Vcompounds, Group IV-VI compounds, Group I-III-VI compounds, GroupII-IV-V compounds, and Group II-IV-VI compounds.
 11. The compositenanoparticle of claim 9 wherein said inorganic semiconductor shell is ofa material selected from the group consisting of Group II-VI compounds,Group II-V compounds, Group III-VI compounds, Group III-V compounds,Group IV-VI compounds, Group I-III-VI compounds, Group II-IV-Vcompounds, and Group II-IV-VI compounds.
 12. The composite nanoparticleof claim 10 wherein said metallic material shell is of a materialselected from the group consisting of cobalt, nickel, iron,iron-platinum, iron oxide, and a magnesium iron oxide spinel.
 13. Thecomposite nanoparticle of claim 1 wherein said nanoparticle includes amagnetic material core of cobalt and an inorganic semiconductor shell ofcadmium selenide.
 14. The composite nanoparticle of claim 1 furhterincluding a spacer material between said core and said shell, saidspacer material selected from the group consisting of dielectricmaterials and inorganic semiconductor materials of a thickness orcomposition so as to function essentially solely as a spacer material.15. The composite nanoparticle of claim 11 further including a spacermaterial between said core and said shell, said spacer material selectedfrom the group consisting of dielectric materials and inorganicsemiconductor materials of a thickness or composition so as to functionessentially solely as a spacer material.
 16. The composite nanoparticleof claim 12 further including a spacer material between said core andsaid shell, said spacer material selected from the group consisting ofdielectric materials and inorganic semiconductor materials of athickness or composition so as to function essentially solely as aspacer material.
 17. The composite nanoparticle of claim 1 furtherincluding a passivating layer upon said shell layer.
 18. The compositenanoparticle of claim 11 further including a passivating layer upon saidshell layer.
 19. The composite nanoparticle of claim 16 furtherincluding a passivating layer upon said shell layer.